The present invention is directed to the manufacture of nitrogen trifluoride (NF3) In particular, the present invention is directed to a process for the reduction or elimination of liquid NH3/HF byproduct during the manufacture of NF3.
The reaction of fluorine (F2) with liquid NH3/HF mixtures (or NH3/HF/MFz mixtures, where M is defined below) produces, NF3, HF, N2 and other nitrogen fluorides as gaseous products and a liquid NH3/HF mixture (or NH3/HF/MFz mixture) as a waste byproduct (the liquid waste melt). The objective of the present invention is to provide a means of eliminating the liquid waste melt produced by the reaction in a reactor. The amount of waste produced per amount of NF3 product varies with the reaction conditions, i.e., pressure, temperature and the molar ratio of HF to NH3 (melt ratio).
U.S. Pat. No. 5,637,285 (Coronell et al.) is directed to a method for the synthesis of NF3 from elemental F2 and a source of NH3 in a gas-liquid reaction comprising 3F2+NH4H(Xxe2x88x921)Fxxe2x86x92NF3+(3+x)HF wherein the melt ratio, HF/NH3, represented in the formula, is at least 2.55 and the reaction liquid is agitated or mixed with a mixing apparatus at a high level equivalent to or greater than 1000 watts per cubic meter. This method allows for nitrogen trifluoride yields of 70% or greater. According to Table 6 in this patent, the amount of waste melt (in pounds) generated per pound of NF3 produced can be as low as 1.47 or as high as 3.5.
The existing technology provides no means of controlling or eliminating the production of liquid waste melt. Until now, the liquid waste melt was either wasted or sold as a low-quality F source.
As indicated above, the reaction of F2 with liquid NH3/HF (or NH3/HF/MFz) mixtures produces NF3, N2, nitrogen fluorides and HF as a byproduct. Some of the byproduct HF escapes with the gaseous products, while the rest is bound by the liquid waste melt in the liquid phase, accumulating in the reactor. The amount of HF escaping with the gaseous products and thus the amount of HF accumulating in the liquid phase is a function of HF vapor pressure above the melt. Filliaudeau and Picard, xe2x80x9cTemperature Dependence of the Vapor Pressures and Electrochemical Windows of the NH4HF2-HF Mixtures,xe2x80x9d Materials Science Forum 73, 669 (1991) provides a relationship that describes the melt HF vapor pressure as a function of temperature and melt ratio. U.S. Pat. No. 5,637,285 (see above) suggests that the reactor process conditions, i.e., agitation, pressure, temperature, vapor and liquid compositions (including melt ratio,) are controlled to optimize F2 conversion and NF3 production yield.
To compensate for the amount of byproduct HF accumulating in the liquid phase and to achieve a constant melt ratio in the reactor, common practice dictates the addition of excess NH3 into the melt, above that required by the NF3 reaction stoichiometry. The excess NH3 binds the byproduct HF that accumulates in the liquid phase and creates excess liquid waste melt, which must be removed.
In Euler and Westrum, xe2x80x9cPhase Behavior and Thermal Properties of the System NH4F-HF, J. Phys. Chem. 65, 1291 (1961), the system NH4F-HF was studied by thermal analysis between the limits NH4HF2 and HF. Euler and Westrum show that NH3 and HF molecules interact strongly, producing mixtures that are liquid at temperatures well above the boiling points of either pure compound. This property makes pervaporation a feasible means of separating pure HF from the melt in accordance with two of the preferred embodiments of the present invention, as described below.
Processes for reducing or eliminating NH3/HF or NH3/HF/MFz liquid waste melt in the manufacture of NF3 are provided. All preferred embodiments include the step of providing an NF3 reactor for a reaction of F2 with a liquid NH3/HF or NH3/HF/MFz mixture that produces NF3, HF, N2 and other nitrogen fluorides as gaseous products and a liquid or NH3/HF or NH3/HF/MFz mixture as the liquid waste melt. For purposes of the present invention, NH3/HF/MFz mixture is an ammonium complex selected from the group consisting of NH4H(Xxe2x88x921)FX, (NH4)yMFz.nHF, and mixtures thereof, where x is equal to or greater than 2.55, y is 1-4, z is 2-8, n is sufficient at reaction conditions to maintain the ammonium complex substantially as a liquid, and M is selected from the group of elements from Group IA through VA, Group IB through VIIB and Group VIII of the Periodic Table of the Elements. Additionally, for purposes of the present invention, the mixture NH3/HF/MFz may include only a mixture of NH3/HF. HF is transferred from the liquid waste melt into a gas liquid or solid medium from which HF can be separated to produce a liquid waste melt that is substantially stripped of HF. The stripped liquid waste melt is transferred back to the reactor. The HF is preferably separated and purified and reused, possibly in an F2 production process.
In a first embodiment, a stripping gas is controllably added to the liquid waste melt to yield a saturated stripping gas mixture that is saturated with HF that has been removed from the liquid NH3/HF/MFz waste mixture and a stripped liquid waste melt. Flow of the stripping gases is metered such that the amount of HF removed by the stripping gases from the liquid waste melt is controlled. The stripping gas may preferably be N2 or NF3 or may also be Ar, He, Ne, N2 and NF3 and the like, but may not include adding NH3 or HF. The process may include the step of transferring the liquid waste melt into an auxiliary stripping vessel prior to the step of adding the stripping gas and may also include the step of exhausting the stripping gas from the stripping vessel whereby the stripping gas is saturated with HF. HF may be separated from the saturated stripping gas mixture and purified and used for F2 production. Purification may be accomplished, for example, by condensation, distillation, adsorption, absorption, or membrane separation. The step of transferring the stripped liquid waste melt from the stripping vessel back into the NF3 reactor may also be included.
In a second embodiment, the liquid waste melt may be transferred from the NF3 reactor into a heat exchanger, where heat is added to the liquid waste melt. The heated liquid waste melt is transferred into a phase separation vessel, which separates the liquid waste melt into a liquid phase product and a vapor phase product. The liquid phase product is cooled in a cooler, and returned to the reactor. The vapor phase product consists of a high percentage HF. The vapor phase product of HF may be purified to yield substantially pure HF and be used for F2 production. The step of purifying the vapor phase product may be accomplished by, for example, condensation, distillation, adsorption, absorption, or membrane separation.
In a third embodiment, the liquid waste melt may be transferred from the NF3 reactor into a pressure controllable vessel where pressure in the vessel is controlled to allow the liquid waste melt to separate into a liquid phase product and a vapor phase product. The liquid phase product may be returned to the reactor, whereby the vapor phase product consists of a high percentage of HF. The process may include the step of purifying the vapor phase product of HF to yield substantially pure HF. The purified HF may be used for F2 production. The step of purifying the vapor phase product may be accomplished by, for example, condensation, distillation, adsorption, absorption, or membrane separation.
In a fourth embodiment, liquid waste melt may be forced through a membrane that selectively allows the passage of HF but not of NH3/HF/MFz complexes, wherein liquid that does not pass through the membrane is added back to the NF3 reactor and liquid that passes through the membrane is substantially pure HF. A step of further purifying the HF that has passed through the membrane may be included. The process may also include the step of using the purified HF for F2 production. The step of further purifying the HF that has passed through the membrane may be accomplished by, for example, condensation, distillation, adsorption, absorption, or membrane separation.
In a fifth embodiment, the process includes the step of absorbing HF from the liquid waste melt using an absorption medium. The process may also include extracting and purifying the HF from the absorption medium. The step of purifying the HF may be accomplished by, for example, condensation, distillation, adsorption, absorption, and membrane separation. The absorption medium may be, for example, metal fluoride/HF complexes (MFz/HF). The step of absorbing HF from the liquid waste melt using an absorption medium may include providing an HF transfer device wherein liquid waste melt is transferred from the NF3 reactor into the HF transfer device in a waste melt stream and wherein the absorption medium is introduced into the HF transfer device in an absorption medium stream, wherein temperatures of the two streams are maintained so that the vapor pressure of the waste melt stream is greater than the vapor pressure of the absorption medium stream. HF will be transferred from the waste melt stream into the absorption medium stream to produce an absorption medium mixture stream. The absorption medium mixture stream may be used for F2 production. The stripped liquid waste melt stream may be returned to the reactor. The HF transfer device may include, for example, two tanks that have connected vapor spaces. Alternatively, the HF transfer device may include, for example, a membrane that allows for passage of HF but not of NH3/HF/MFz.
Finally, a sixth embodiment of the present invention may use a combination of the above five embodiments with optimization of the system. The first through fifth embodiments can be combined, optimized and operated so as to achieve transfer of HF from the liquid waste melt into a solid, liquid or gaseous medium, from where HF can be easily recovered, purified and reused, possibly in an F2 production facility.